"After we discovered we could not take care of Gaza tunnels with airstrikes alone, we launched a ground operation," Prime Minister Benjamin Netanyahu said Friday. "My direction to the [military] is to prepare for a possible substantive broadening of the ground operation, and they are ready."

Sending in ground troops raises the stakes considerably. It puts the Israeli forces at far greater risk — one soldier was killed in the early hours of the ground invasion. It also exposes Israel to increased international criticism and raises questions about whether any temporary military gains will translate into a sustained period of calm.

Here are several key things to watch as the Israeli ground campaign plays out:

Palestinian rocket fire: There's a broad Israeli consensus that Israel should hit back hard when it comes under attack. The heavy Palestinian rocket fire out of Gaza has again placed great domestic pressure on the country's leaders to stop it immediately.

Northern Gaza has been the main launching ground for the Palestinians since they began firing their crude, homemade projectiles in the early 2000s. The rockets are now more sophisticated and have much greater range, though most are still unleashed from the north and directed at towns and cities in southern Israel, particularly Sderot.

As expected, Israel's forces entered Gaza in the north. The territory is only 6 miles wide at the northern end and the Israeli forces have a track record of tamping down the fire when they go in on the ground.

In a well-established pattern, the Israeli incursions are followed by a period of relative calm, but it eventually gives way to renewed rocket fire. Given that the Palestinians are much better at making rockets, it's not clear how Israel will prevent Palestinians from rebuilding their arsenal.

Palestinian smuggling tunnels:Most of the tunnels are at the opposite end of Gaza, on its southern border with Egypt, and Palestinians have used them for as long as anyone can remember to bring in weapons, cigarettes, food, clothes and even zoo animals.

However, the current Israeli operation is focused on tunnels that lead from the eastern edge of Gaza into Israel, which would allow the Palestinians to carry out an attack in Israel or possibly kidnap an Israeli soldier or civilian and take them back into Gaza.

The Israeli military said troops have found 10 tunnels with 22 separate exit points from Gaza since the ground operation began Thursday.

The tunnel system expanded dramatically after Israel pulled its military and its settlers out of Gaza in 2005. The tunnels appear to be more vulnerable today than in the past.

Egyptian ruler Abdel-Fattah el-Sissi has no love for Hamas, viewing the group as an extension of the Muslim Brotherhood in Egypt. Therefore, Egypt has cracked down on the tunnels. In turn, Israel sees this as an opportune moment to seal off tunnels on the Gaza side.

Israel was never able to close down all the tunnels even when it had a large military force based in Gaza from 1967 to 2005. But it could greatly disrupt the tunnel network and limit the Palestinians' ability to import weaponry.

A weak and isolated Hamas:Hamas has dominated Gaza since winning the Palestinian election in 2006 and driving out the rival Fatah movement a year later. But with Israel squeezing Gaza, Hamas has never been able to effectively run the impoverished territory and the group has been struggling on several fronts.

Hamas recently relinquished some political power in a unity deal with Mahmoud Abbas, the Palestinian leader in the West Bank. Hamas' main patrons, Syria and Iran, are preoccupied with problems of their own. By most estimates, the group's standing among Palestinians has declined.

Still, Hamas has survived previous Israeli incursions and remained the leading force in Gaza.

"Netanyahu is killing our children and will pay the price. The ground invasion doesn't frighten us and the occupation army will sink in Gaza's mud," said Hamas spokesman Sami Abu Zuhri.

One key question is whether Israel will go after Hamas' top political leaders. In general, Israel has focused on militants responsible for the rockets and other attacks. Top political figures, like Ismail Haniya, who leads Hamas in Gaza, have generally not been targeted.

But Israel has gone after top Hamas leaders at times. In 2004, Israel killed the top figure, Sheik Ahmed Yassin. His successor was killed in another Israeli airstrike a couple of months later. Hamas leaders are deep in hiding with Israeli troops in Gaza, and Israel may not be able to find them even if it is looking for them.

The question of Gaza City:With 1.8 million Palestinians crammed into the small seaside enclave, all of Gaza is crowded. This is especially true of Gaza City. If Israeli troops enter the city, they could be extremely vulnerable to ambushes as they operate armored vehicles in narrow streets while Palestinians can fire on them from high-rise buildings. Israel suffered high casualties when it entered Gaza City a number of times in the early 2000s and has generally stayed out of the city since then.

Israel's exit strategy: Both sides know this is a temporary incursion and will not resolve any larger political questions since Israel and Hamas don't negotiate with each other. They also know Israel can advance at will and take whatever positions it likes.

However, once in place, Israel faces the problem encountered by all occupying forces: As troops settle in, they become sitting targets. Israel presumably wants to get in and out as quickly as possible. The longer the incursion drags on, the more it suggests Israel is not accomplishing its military goals, the more vulnerable its troops become and the more international criticism it is likely to face.

In the past, cease-fires have been brokered by Egypt, but the fundamental questions in the dispute have gone unaddressed.

Greg Myre, the international editor of NPR.org, covered the Israeli-Palestinian conflict from 1999 to 2007 and is the author ofThis Burning Land. You can follow him@gregmyre1

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